In an office in Cardiff, a scientist brings his concerns to the Mayor Margret Blaine over a new nuclear power plant to be built there. It is dangerous, almost as if it had been intentionally built to explode. Blaine asks him if he has told anyone else about his findings. He replies that he did not, and instead went directly to her. She commends him for making the right choice — as she apparently and audibly experiences some gas. As the scientist expresses his relief that Blaine will shut down the project, she reveals herself to be a Slitheen, and kills the scientist.

The Ninth Doctor has landed the TARDIS over the Cardiff rift located in the Roald Dahl Plass, using slow radiation leakage to recharge the TARDIS. As the process will take a whole day, he, Rose, and Jack are joined by Mickey Smith and take the opportunity to explore the area. While they enjoy a meal at a restaurant, the Doctor notices to his dismay the front page of the Western Mail, with the headline "New Mayor, New Cardiff" and a picture of Blaine, whom they known as the human form of Blon Fel-Fotch Passameer-Day Slitheen whom they previously had encountered. Since their meeting, Blon has become the Lord Mayor of Cardiff, and initiated the construction of a nuclear power plant. However, several people had found significant flaws in the design that could lead to a nuclear meltdown, and had approached her about these issues, but they have since disappeared, Blon having killed them herself. During a press conference, a young reporter named Cathy Salt approaches Blon about these deaths and the information they had left behind. Blon thinks she should have a word in private; a loud rumble in her belly gives her an excuse to go to the toilet and she takes Cathy with her. Disgusted as she hears Blon on the toilet, Cathy notes they got there just in time. Blon gets out of her skin suit and plans to kill her, but has a change of heart as Cathy talks about her family and unborn child, realising that she herself no longer has one.

Realising that they must stop Blon, the Doctor's group converges on City Hall and eventually capture Blon after chasing her through repeated uses of a teleporter. She tells the group that the teleporter is how she escaped the destruction of the rest of her family, and that she hopes that, as planned, the meltdown of the plant would open the Rift and destroy the planet, with her using a hidden tribophysical waveform macro-kinetic extrapolator — a pan-dimensional surfboard — to escape the explosion. The Doctor notices that the name of the plant, Blaidd Drwg, is Welsh for "Bad Wolf", a phrase that he has observed before in his adventures with Rose. The Doctor tells Blon he will take her back to her home planet of Raxacoricofallapatorius, but Blon notes that the Slitheen family are convicted criminals there and she will be executed, which the Doctor insists is not his problem.

A last meal and attempted murder.

Jack recognises that the extrapolator can be used to halve the time to refuel the TARDIS, and stays there to install it. Rose and Mickey go out for a drink to discuss their relationship; Mickey, while inviting Rose to a hotel room, claims to be seeing someone else since Rose is not there for him. Rose counters that she knows the woman, that Mickey doesn't even like her and that "that's never gonna happen, so who do you think you're kidding?" She argues that this conversation has nothing to do with Trisha. Mickey says that he can't even go out with a girl from "the shop" because Rose picks up the phone to say she's coming back to present-day Earth and Mickey comes running for her. When Mickey claims he'd wait for Rose for the rest of his life, Rose apologises.

At the request of Blon, the Doctor joins her for one last meal at her favourite restaurant, equipped with bracelets that will electrocute Blon if she gets more than ten feet away from the Doctor. Blon attempts to kill the Doctor through various means, but the Doctor is able to casually block the attempts. Blon then attempts to gain the Doctor's sympathy, bringing up her childhood and her last-minute change of heart over killing Cathy. Though the Doctor dismisses her act of kindness as a way of living with herself, he does sympathise. Before he can agree to take her elsewhere, however, a large earthquake shakes the area.

Blon gets her wish.

The group reassembles in the TARDIS, where a bright column of light is shooting up overhead. Jack tells the Doctor that it is the power from the Rift, drawn by the extrapolator. Blon reveals that this was her plan all along — the extrapolator would have been found by someone of sufficiently advanced technology to recognise the Slitheen, and would have activated it, causing it to lock onto the nearest alien power source (the TARDIS in this case), to tear open the Rift and eventually the Earth, while she would have still ridden the device to escape the destruction. Blon takes Rose hostage, choking her, and demands the extrapolator, or Rose will die. The Doctor warns her that this isn't just any ship her device has latched upon - this is the TARDIS. Before she can use the extrapolator, the heart of the TARDIS opens and shines in her face; Blon dreamily looks into the light with a smile, and then beams at the Doctor, emphatically telling him, "Thank you." The light overtakes her, and shortly her skin suit falls empty to the console floor. The Doctor manages to close the TARDIS console and reseal the Rift once more. When they investigate the suit, they find a Slitheen egg; the Doctor surmises as the TARDIS is telepathic, it may have sensed that Blon wanted a second chance and gave that to her. As the Doctor, Rose, and Jack prepare to travel to Raxacoricofallapatorius to deliver the egg, Rose realises that Mickey has left; the Doctor offers to wait for him, but Rose lets him go, allowing him to also have a second chance.

Not every person who worked on this adventure was credited. The absence of a credit for a position doesn't necessarily mean the job wasn't required. The information above is based solely on observations of the actual end credits of the episodes as broadcast, and does not relay information from IMDB or other sources.

An early Radio Times synopsis of this episode read "The Doctor comes across someone he thought was long dead...". Many viewers thought this to be in reference to a classic series villain, usually the Master. This was ultimately proven untrue, as the reference was to Blon Fel-Fotch Passimeer-Day-Slitheen.[source needed]

Torchwood Three had been in existence since at least 1899 in Cardiff, with Jack involved in the organisation since 1899 and in charge from the end of 1999. (TV: Fragments) This means that the Doctor's arrival near the rift, his party's reunion with Mickey, and the subsequent Slitheen adventure — all involving Captain Jack — take place very close to the older Captain Jack and his Torchwood team. Jack would later reference having to wait until a version of the Doctor that coincided with him arrived; encountering the Doctor and his younger self at this point would have been unacceptable. (TV: Utopia) In addition, a third, older Jack, cryogenically frozen, is also nearby, unknown to both younger Jacks. (TV: Exit Wounds) In PROSE: The Twilight Streets, it is established that Jack locked himself and his team inside the Hub for the day to prevent them from meeting his younger self.

A newspaper clipping regarding Blaine's election was pinned up in the Torchwood Three "information office" entrance. (TV: Reset)

Similarly, another enemy of the Doctor, Pangol, was also regressed in time (this time to a baby). (TV: The Leisure Hive)

In his fifth incarnation, the Doctor was similarly told by an alien that returning them home would result in their death, but that time he offered to take the criminal to another planet to avoid his death sentence. (TV: The Visitation)

Contrary to common belief, season 10 kicked off in the last week of December 1972 — not in 1973, as would be expected. Season 10 actually began nine years after season 1 started. In fact, The Three Doctors began nine years to the week after The Daleks first aired.

For the purposes of this list, "Series 4" is considered to be the production series 4, which ran all the way from Time Crash to The End of Time.

The years seen in this section may seem decidedly "off". Remember, however, that this list only gives the first year in which an episode from a series was broadcast. David Tennant, unusual amongst other Doctors, began and ended on special episodes, not regular ones. Thus, his series actually begin in 2005, 2006 and 2007 — not 2006, 2007 and 2008 as is commonly thought.